1. Field of the Invention
This invention relates generally to an electronic system module and method of fabrication, and more particularly to an electronic system module fabricated on a carrier from which it can be released.
2. Description of the Related Art
The number of input/output (I/O) connections required by integrated circuit (IC) chips is increasing, to several hundred for recent microprocessor chips. As verification of complex designs becomes an increasing portion of the total design activity, it is desirable to increase the I/O count further, to provide access to more internal nodes for testing. Flip chip assembly methods have helped to provide more I/O connections because they provide an area array of connections across the entire face of an IC chip, rather than just at the perimeter as with wire bonding. However, it continues to be desirable to reduce the pad pitch, the distance between bonding pad centers, in order to achieve more I/O connections per unit area of IC chip.
A recent advance in flip chip assembly capability has been the introduction of stud bumping machines that can provide gold stud bumps on IC chips with pad pitches of less than 100 microns. However, to take advantage of this capability, the system board that receives the bumped devices must have fine traces in order to route all of the signals with space efficiency, and to support bonding pitches less than 100 microns. The most recent packaging technology to be commercially introduced is called land grid array, LGA. It builds up the wiring layers by plating a base layer of copper that has been patterned with photo resist. The external terminal pitch claimed for this packaging method is “less than 0.5 mm”. None of the available printed circuit board technologies can support direct mounting of bumped devices at a pitch of 100 microns or less. The current invention is capable of pad pitches of less than 100 microns, including a viable method for reworking defective IC chips at this bonding density.
For many years the minimum trace width available from printed circuit board vendors has been around 100 microns. Recently, advanced multi-layer circuit processes have achieved trace widths of 17 microns. The current invention is capable of achieving trace widths of 5 microns or less, together with a trace pitch of 10 microns or less.
One way to achieve fine line interconnection circuits is to employ a semiconductor fabrication facility and to build the interconnection circuit on a silicon wafer; hence the term, wafer level packaging, WLP. The precision of the associated photolithographic methods, the clean room environment with low particulate count, and the advanced substrate handling equipment of such a facility can all contribute to high-density interconnection circuits. However, the application of IC chip manufacturing facilities to this problem is more than what is required. An intermediate alternative is to apply the manufacturing resources of a glass panel fabrication facility, where the minimum feature sizes are 10 to 20 times larger than for IC chips (but still adequate for the most advanced assembly processes), and the manufacturing cost per unit area of devices produced is less than 5% of the cost per unit area of IC chips. In addition, the glass panel fabrication facility can produce system boards of any size up to approximately a meter square, whereas the largest wafers produced have a diameter of 300 mm. In order to avoid the rigidity and weight of the glass substrate, and to provide better thermal access to the heat producing components for cooling them, it is desirable to discard the glass carrier after most of the processing is done.
Typically, WLP has used redistribution circuits to map from the fine pitch available with flip chip bonding to the coarser pitch of a printed circuit board. The current invention eliminates the redistribution circuits because the printed circuits produced (termed interconnection circuits) include fine features that easily accommodate the fine pitch of the flip chip bonding.
It has been common practice to produce printed circuit boards at one facility, and perform system assembly and test at another facility. However, there are major advantages to integrating the circuit board manufacturing process with the assembly and test process to create a single, unified, fabrication, assembly, and test process. One advantage is in reducing the time to develop and debug a new design. Flexibility in the proposed process allows adaptation to component and assembly yield problems as they arise (as each additional component is assembled), providing more detailed testing sequences as necessary. For example, during prototype testing, components may be assembled onto the circuit substrate one at a time, providing a test environment of a partial system, and making the minimum change of a single IC chip between one test and the next. The test software can be adapted to address detailed issues as they arise. Once confidence has been achieved at this level, components may be assembled and tested in functional groups as the product moves into production. A tighter integration of personnel is also achieved because all of the variables are controlled in one place. This flexibility, wherein the assembly process is tailored to yield issues in real time, is not available with conventional testing methods. Usually, an entire system is assembled before any system testing is performed. The current invention employs incremental system level testing, as each component is attached to the circuit assembly. The testing of each component is performed fewer times, because the incremental assembly and test process essentially guarantees system integrity at each step. This contrasts with conventional methods requiring component test, sub-assembly test, and system test, with numerous iterations if problems develop.
More accurate and complete testing of components is provided when they are tested in their system environment, rather than individually. The system environment is created with the actual system, or a subset thereof, rather than a simulated environment created by test vectors programmed into a general-purpose tester. This can lead to lower test cost and faster test development, by eliminating the need to generate and debug detailed system response patterns. If the system level requirements are satisfactorily met, then the minutiae of component level characteristics become irrelevant. Alternatively, only the functions relevant to proper system function are tested; this is a much more manageable set of requirements than the total set of functions that all the assembled components are capable of performing.
The signal voltage swing is reducing with each new generation of IC chip technology. This makes it more difficult to test remotely through a cable, and still achieve the necessary noise margins. Providing test chips on the motherboard will provide shorter trace lengths for testing, which will be more robust with respect to both timing issues and noise margin.
Each component may be verified at an elevated system temperature before attaching the next component. This can be accomplished by heating the glass carrier underneath the circuit assembly. By providing a predetermined test temperature to the entire circuit assembly, a speed grade can be associated with the module, as has been done in the past at the component level. Greater emphasis can be placed on environmental stress testing at the system level. Also accelerated life testing can be performed early in the life cycle of a product, and lessons learned about particular components can be incorporated into the system level test. The only tests performed on each component correspond to system level requirements; by not testing component requirements that are irrelevant to the particular system, higher system yields may be achievable.
The physical structure of the system module of the current invention can lead to other conveniences. An example relates to accelerated life testing of a module. Because the current invention allows a sophisticated system (circuit assembly) to be implemented in a module of small size, and because the system module packaging includes a metal envelope surrounding the module that provides good heat distribution, temperature control can be achieved by placing the module on a hot plate rather than in a bulky and inaccessible environmental chamber.
Hermetic packaging techniques and electromagnetic shielding techniques can be applied at the module level to improve both performance and manufacturing cost. Performance is improved because a single metal envelope encloses almost the entire module, avoiding the interference from individual components and the wiring between them. Cost is reduced because hermeticity and shielding are provided with a simple process applied once to the entire system, rather than being addressed individually at each of the components.
Such a unified process has only recently become feasible. It depends on using common semiconductor manufacturing equipment to support fabrication of the interconnection circuits, bonding sites, test connection fixtures, module cables, and the module layers that create hermeticity and electromagnetic shielding. It also depends on the fact that sophisticated and programmable IC chips can now implement the testing function across all of the components in a system, including digital, analog, and RF functions, if multiple IC chips are employed for testing. Adding these functions to the system using the current invention is not as expensive as in the past, because the packaging and assembly cost is minimal. Preferably, a tester is included with every module produced, but the cost of the tester is small compared with the system level assembly and performance benefits, and the reduction in system development time.
R. K. Traeger, “Hermeticity of Polymeric Lid Sealants”, Proc. 25th Electronics Components Conf., 1976, p.361, has documented the water permeabilities of silicones, epoxies, fluorocarbons, glasses and metals. Traeger's data shows that, in terms of providing a barrier to water, a layer of metal that is 1 micron thick is approximately equivalent to a layer of glass that is 1 mm thick, and also equivalent to a layer of epoxy that is 100 cm thick.
Generally flip chip bonding techniques require an epoxy under-layer between flip chip mounted IC chips and the circuit board. The purpose of the under-layer is to provide mechanical strength to withstand repeated thermal cycling without developing cracks in the area of the flip chip bonds. The thermal stress arises because of the difference in thermal coefficients of expansion (TCEs) between the IC chip material and the board material. Gelatinized solvents have typically been used to dissolve the epoxy; they leave a residue that must be cleaned off. The process of cleaning off the residue has typically resulted in damage to the fine pitch bonding sites, to the point where they cannot be reliably re-bonded. The under-layer is unnecessary with the current invention because the flexibility of the final interconnection circuit substantially eliminates thermally induced stress in the region of the flip chip bonds. Without the thermally induced stress, no cracking will occur. Thermal stresses are still present during assembly (because the interconnection circuit is rigid at this point), but are avoided during operation in the field (when the interconnection circuit is flexible). The number and extent of thermal cycles during assembly are more predictable and controllable than the thermal cycles arising from operation in the field. Stress testing in the laboratory can be used to quantify the acceptable temperature limits, and assure crack-free circuit assemblies. Avoiding the under-layer makes a robust rework process possible. This general concept is referred to in the art as compliant packaging technology. A related issue is the recent requirement for low stress in IC chips that use ultra-low-k dielectric materials. IC chips attached to flexible substrates will experience low stress and will be accommodating of the ultra-low-k dielectrics.